Myeloperoxidase (MPO; EC 1.11.1.7) is a tetrameric, heavily glycosylated basic heme protein of approximately 150 kDa. It is composed of two identical disulfide-linked protomers, each of which possesses a protoporphyrin-containing 59–64 kDa heavy subunit and a 14 kDa light subunit. U.S. Pub. No. 2002/0164662; Hoy et al., Clin. Chem. Lab. Med. 40: 2–8 (2002). In vivo, MPO converts Cl− via a two-electron peroxidation step into hypochlorous acid, HOCl, a powerful oxidizing agent capable of destroying microbes. Marquez et al., J. Biol. Chem. 265: 5666–5670 (1990).
MPO plays an important role in host defense against invading microorganisms. MPO is abundant in neutrophils and monocytes, accounting for 5% and 1 to 2% of the dry weight of these cells respectively. Marquez et al., J. Biol. Chem. 265: 5666–5670 (1990); U.S. Pub. No. 2002/0164662.
MPO is implicated in a large spectrum of diseases. Besides participating in the defense against microorganisms via the production of HOCl, MPO is released in inflammatory states where migrating neutrophils may release active enzyme. Hoy et al., Clin. Chem. Lab. Med. 40: 2–8 (2002). Increased MPO levels have been reported in infections and anti-MPO antibodies accumulate in systemic vasculitites. MPO is also involved in non-infectious diseases, such as atherosclerosis, cancer and promyelocytic leukemia, neurodegerative diseases including Alzheimer's disease and multiple sclerosis. Hoy et al., Clin. Chem. Lab. Med. 40: 2–8 (2002).
MPO mRNA is widely used in clinical chemistry as a marker for acute myeloid leukemia (AML). Bennett et al., Br. J. Haematol. 33: 451–8 (1976). Higher expression genotype of the MPO G-463A polymorphism has also been reported to be related to AML. Reynolds et al., Blood 90: 2730–7 (1997). The MPO G-463A polymorphism characterized by a G/A transition is located with Alu sequences of a promoter region containing a hormone response element. The G/G genotype has been related to increased MPO expression and protein level in cells of leukemic patients. Reynolds et al., Blood 90: 2730–7 (1997). It has also been shown that subjects homozygous for the A allele are at a decreased risk for lung cancer. London et al., Cancer Res. 57: 5001–3 (1997); Le Marchand et al., Cancer Epidermiol. Biomarkers Prev. 9: 181–4 (2000); Cascorb et al., Cancer Res. 60: 644–9 (2000); Schabath et al., Carcinogenesis 21: 1163–6 (2000). However, a recent study shows that the A allele is associated with an increased risk of lung cancer among a subset of older men. Misra et al., Cancer Lett. 164: 161–7 (2001).
MPO is present in the microglia in the brain of patients with multiple sclerosis (MS) and in the microglial cells surrounding senile plaques of cerebral cortex from Alzheimer's disease (AD) cases. Jolivalt et al., Neurosci. Lett. 210: 61–4 (1996); Nagra et al., J. Neuroimmunol. 78: 97–107 (1997). An alternation of MPO level is also related to atherosclerosis and brain infarction. It has been reported that MPO/H2O2/Cl− system is one of the possible mechanisms involved in the initiation of atherosclerotic lesions. Dautherty et al., J. Clin. Invest. 94: 437–44 (1994). Heinecke et al. Curr. Opinion Lip. 8: 268–74 (1997); Hazell et al., J. Clin. Invest. 97: 1535–44 (1996); Malle et al., Eur. J. Biochem. 267: 4495–503 (2000). One of the main consequences of atherosclerosis is brain infarction and measurement of MPO activity is a widely used marker of neutrophil infiltration of the brain parenchyma. Barone et al., J. Neuroscie. Res. 29: 336–45 (1991). Increased MPO activity has been observed in the serum of patients after an ischemic brain infarct. Re et al., Eur. J. Emerg. Med. 4: 5–9 (1997). Measurement of MPO has also been used for predicting cardiovascular risks. Brennan et al., N. Engl. J. Med. 349: 1595–1601 (2003); U.S. Pub. No. 2002/0164662.
The broad range of diseases MPO is implicated in, and the possibility of using MPO as a clinical marker and therapeutic target, make assays for accurately measuring MPO levels and activities invaluable. Several assay methods have been reported. WO 02/090575, U.S. Pat. No. 6,022,699. However, there is still a need for a reliable and sensitive method for assaying MPO.